MODELING SURFACE-WAVES IN ANISOTROPIC STRUCTURES-II - EXAMPLES

Surface waves in several anisotropic geological settings are studied b y numerical simulation. Asymptotic point-source synthetic seismograms are obtained using formulas from a companion paper, the group velociti es and mode-excitation factors being evaluated via variational integra ls. Some alternative waveform approximations and ray-path effects are explored and the possible significances of peculiarities of anisotropy , such as wavefront folding, are at least partially assessed. Model on e is a 5-layer oceanic environment with +/-5.6% aligned vertical-crack S-wave anisotropy in the upper crust, The 2-3 s waveforms often have highest group velocities at 45 degrees to the cracks and there are ver y strong transverse components from a vertical source. Maslov and ray/ WKBJ waveforms agree well, even though for some modes and frequencies azimuthal group-velocity variations imply the latter is suspect. The M aslov-integral end-point signals mean that it too requires care and th e so-called quasi-WKBJ approximation is a viable alternative. Periods approaching 1 s and/or high crack densities lead to very complicated g roup-velocity geometry indeed. The second model has +/-2% upper-mantle mineral-alignment anisotropy, with its isotropic component constraine d to agree with model ak135. The 20-30 s waveforms from a transversely -isotropic source force have fundamental 'Rayleigh' and 'Love' packets showing clear azimuthal phase anomalies but surprisingly standard pol arizations and amplitudes, Only higher modes show such variable amplit udes and phases that they might be identified as likely due to anisotr opy. Around 10-s period the higher-mode family has an interesting band ed group-velocity structure and as the period is lowered further, two modes separate out and the band tightens in what may be a characterist ic feature. Two candidate models of mantle-plume anisotropy show disti nct azimuthal dependencies of mode velocities. Point-source surface-wa ve rays tracked in one plume model display shadows and focusing and sh ow how initially folded wavefronts can unfold in the lateral variation s. The upper-mantle models are numerically taxing. R/T coefficients an d mode slownesses can be found using double-precision arithmetic, but depth profiles and variational-integrals sometimes require greater acc uracy. We find that while displacements near the stack top and bottom may be obtained quite easily, as frequency increases it becomes increa singly difficult to compute those at mid-depth. We find no reason to b elieve that the Kennett invariant-embedding scheme has lesser accuracy than others. (C) 1997 Elsevier Science B.V.